Light guide device enhancing a polarized component and liquid crystal display device

ABSTRACT

A polarized component is obtained with a high conversion efficiency in a light guide which produces one of the polarized components by having it transmitted. The light from a light source is incident to a light guide which comprises a plurality of light guide layers and reflected by the end surface to an interface between the light guide layers. The polarized component transmitting through the end surface is rotated in its polarization plane by a wave length plate and reflected by a reflecting plate for reentrance to the light guide at the end surface of the light guide toward the interface. The reentering light mostly transmits through the interface because the polarization plane is rotated. A reflected light polarized component is returned to the wave length plate and the reflecting plate, and directed back to the interface again. The polarized component transmitting through the interface is similarly transmitted and reflected in the next interface. The number of interfaces can be reduced by increasing the reflection of the polarized component reflected in the interface. For this purpose, the index of refraction in the direction along the axis of the reflected polarized component is increased by making the index of refraction of the light guide layer anisotropic.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a light guide unit for use in aliquid crystal display device in which a polarized component of light isenhanced and a liquid crystal display device which is provided with suchlight guide unit. Particularly, this invention relates to a light guideunit for efficiently converting the light from a light source to apolarized light and a liquid crystal display device having means forefficiently directing-the polarized light emitted from such light guideunit to a liquid crystal cell.

[0003] 2. Description of the Related Art

[0004] A liquid crystal display device is conventionally observed bydirecting polarized light to a liquid crystal cell to cause thepolarization plane to be rotated depending on the condition of the cellfor passage through a polarizer plate. A light source of the polarizedlight is placed in the back of the liquid crystal plate and thus iscalled a “back light”. For obtaining such polarized light wave, anon-polarized light was conventionally incident to a polarizer plate andeither one of the polarized components; i.e., S component and Pcomponent, was absorbed.

[0005] Assuming that a plane defined by a light incident to a point ofincidence on a surface is an incident plane, a polarized componentparallel to the incident plane is called a P component while a componentperpendicular to the incident plane is called an S component. Therefore,more than 50-percent of an incident light was not effectively utilizedin principle and an actual measurement shows that about 58-percent ofthe incident light is absorbed.

[0006] Further, a light dispersing sheet having printed dots wastypically used in addition to a polarization device for obtainingpolarized light by absorbing a polarized component in a conventionalLiquid Crystal Display (LCD) device, and this makes an additional20-percent of the light unavailable.

[0007] In FIG. 1, a LCD module 100 of a conventional LCD device isshown. The light emanating from a light source 101 transmits through alight guide plate 102 having 96% transmittance, a dispersion sheet 103having 80% transmittance, a lower polarizer plate 104 having 42%transmittance, a glass substrate 105 having a numerical aperture of 40%,a color filter 106 having 30% transmittance, and an upper polarizerplate 107 having 90% transmittance, resulting in an actually availablelight intensity which is 3.5% of the light generated in the light source101. This greatly prevents the energy from being utilized efficiently.

[0008] A back light system of a high intensity for use in a low powerconsumption LCD device is especially desired because it is an importantobjective in a portable personal computer to assure a longer usable timewith a given capacity of a battery and the power consumption of a backlight 108 is a major percentage of total power consumption.

[0009] Also, the light energy absorbed in the lower polarizer plate 104,etc., is converted to heat energy which contributes to degradation ofparts of the LCD device. Particularly for a liquid crystal material ofSTN (Super Twisted Nematic) type in which the display quality isdegraded by heat, it is an important objective to reduce such heatgeneration. As seen from FIG. 1, 66.4% of the light energy is convertedto heat energy by the light absorption in the lower polarizer plate 104and the dispersion sheet 103 (this is 69% of heat generation by thelight energy).

[0010] In order to solve such problems, the applicant of thisapplication filed Japanese patent application no. 9-249139 relating to amethod of improving the efficiency of light utilization in obtaining apolarized light by making available for use at least a part of apolarized component which had not been utilized. The principle of thismethod is shown in FIG. 3.

[0011] Light from a fluorescent lump CFL which is a light source isincident to the end surface of a laminated light guide plate unit via areflecting mirror and a collimator. It propagates through the layers ofthe light guide plates, and arrives at the other end surface which iscut in an angle. The incident light is partly reflected at the other endsurface with the rest being transmitted therethrough. The polarizationplane of the light transmitting through the end surface is rotated by aquarter wave length plate placed thereunder and reflected by areflecting plate placed under the quarter wave length plate forreentrance to layers of the light guide plate again through the quarterwave length plate as a P component.

[0012] The P component reentering the light guide plates is incident tothe interface with an adjacent light guide plate layer. The angle ofincidence of the light on the interface is the Brewster angle (to bedescribed later in detail). Therefore, all the P component and a part ofthe S component of the light incident to the interface transmit throughthe interface with the rest of the S component reflected back to thequarter wave length plate and the reflecting plate. The light reflectedagain by the reflecting plate is again directed to the interface afterbeing converted to a P component by the quarter wave length plate whereall the P component and a part of the S component, if any, transmit withthe rest being reflected.

[0013] The light reflected here is reflected repeatedly in a similarmanner and a light converted to a P component for each reflectiontransmits through the interface. As such, the light guide unitultimately emits a large portion of the light from the light source as aP component. The polarized light is emitted in the direction largelydeviated from the normal to the front. A prism sheet for redirecting thelight to the front toward the liquid crystal cell is used. Thepolarization can be further improved by placing a further polarizationplate on the prism sheet.

[0014] Because the reflectance and transmission characteristics aredifferent between the S component and the P component, the lighttransmitting through the interface and the light reflected by theinterface have different polarization components. To explain theprinciple of operation of this invention, a change of polarizationcomponents of the light in transmitting through or reflecting from theinterface between materials of different indices of refraction isdescribed with reference to FIGS. 4, 5 and 6.

[0015] In FIG. 4, when light 204 reaches an interface 203 between twomaterials 201 and 202 having different indices of refraction n₁ and n₂,respectively, a part of the light 205 is reflected when the angle ofincidence φ₁ is less than a critical angle while a part of the light 206transmits through the interface. Assuming that a plane defined by alight incident to a point of incidence on a surface is an incidentplane, the incident light 204 is divided into a P component parallel tothe incident plane and an S component perpendicular to the incidentplane.

[0016] Modifying Maxwell equation for a dielectric material, thetransmittance of the polarized components P and S are given by;

Tp=sin (2φ₁)×sin (2φ₂)/(sin²(φ₁ 30 φ₂)×cos²(φ₁−φ₂))

Ts=sin (2φ₁)×sin (2φ₂)/sin² (φ₁+φ₂)

n ₁ ×in (φ₁)=n ₂×sin (φ₂)

[0017] where

[0018] Tp: transmittance of P component (1−reflectance Rp)

[0019] Ts: transmittance of S component (1−reflectance Rs)

[0020] φ₁: incident angle of light

[0021] φ₂: exit angle of light

[0022] n₁: index of refraction of material 201

[0023] n₂: index of refraction of material 202

[0024] or it is known that;

Rp=((n ₁/cos φ₁ −n ₂/cos φ₂)/(n ₁/cos φ₁ +n ₂/cosφ₂))²

Rs=((n ₁×cos φ₁ −n ₂×cos φ₂)/(n ₁×cos φ₁ +n ₂×cos φ₂))²

[0025] where

[0026] Rp: reflectance of P component (1−transmittance Tp)

[0027] Ts: reflectance of S component (1−transmittance Ts)

[0028] The reflectance of the P polarized component and S polarizedcomponent vary depending on the incident angle φ₁ and the exit angle φ₂as shown in FIG. 5 and FIG. 6, and differ from each other even in a sameincident angle φ₁ (reflectance/transmittance characteristics aredifferent between S and P polarized components).

[0029] For example, when the light proceeds from an acrylic materialhaving an index of refraction of 1.49 to air which has an index ofrefraction of 1.00 (FIG. 6), the critical angle in which a totalreflection takes place is 42.1-degrees. If the light is incident at40-degrees which is less than the critical angle, the exit angle φ₂ willbe 77.8-degrees according to Snell's law. Substituting the aboveequation of Rs and Rp with this, the reflectance for the S component is35.69% while the reflectance for the P component is 7.98%.

[0030] It should be clearly understood from the above descriptionreferring to FIGS. 4 to 6 how the polarized components of the light aretransmitted and reflected in the interface in this invention.

[0031] It is understood from the above-described principle that it isimportant for the layers of the light guide to be laminated in multiplelayers to cause the unnecessary S component to be reflected back eachtime the light reaches the interface between the layers and to bereturned as a P component for transmitting through the interface therebyimproving the efficiency of converting the light emitting from the uniteventually to a P component.

[0032] However, it is disadvantageous to laminate too many layers fromthe view point of the efficiency of utilizing the energy of the lightsource because each layer invites some loss of light. In addition, theincreased number of laminated layers would result in the increase of thethickness of the entire unit even if a thin layer is used. The increaseof the thickness would also invite an increase of the weight. It is themost important objective for a portable information processing device,such as a notebook computer, to decrease the power consumption of itsbattery as well as the thickness and the weight of the entire unit asmuch as possible.

SUMMARY OF THE INVENTION

[0033] This invention relates to an improvement of a light guide unit ofthe above-described type, and it is an object of this invention toprovide a light guide unit having an unchanged performance with adecreased thickness of the entire unit.

[0034] It is another object of this invention to improve the brightnessof a liquid crystal display device without resulting in an increase ofpower consumption by efficiently combining the polarized light from suchlight guide unit of a high efficiency to a liquid crystal cell.

[0035] The basic configuration of this invention lies in a structure inwhich the light from a light source incident to an end surface of a unitof laminated light guide plates propagates through each layer of thelight guide plates and is partly reflected by the other end surfacewhich is obliquely cut. The rest of the light transmitting therethroughcauses the polarization plane of the transmitting light to be rotated bya wave length plate lying thereunder and reflected by a reflecting platelying under the wave length plate for reentrance to the light guideplate again through the wave length plate as a P component.

[0036] The P component reentering the light guide plate is incident toan interface between neighboring light guide plates. The incident angleof the light incident to the interface is adapted to be the Brewsterangle. Therefore, all P component light incident to the interface and apart of the S component light transmit the interface while the rest ofthe S component light is reflected back to the wave length plate and thereflecting plate. The light reflected again by the reflecting plate isdirected back to the interface after being converted to a P component bythe wave length plate, and all P components and a part of S component,if any, transmit through the interface while the rest is reflected.

[0037] The light reflected here is subject to the same processrepeatedly, and a light converted to a P component in every repetitiontransmits through the interface. As such, the light guide uniteventually emits a large portion of the light from the light source as aP component. Because the polarized light is emitted in the directionlargely deviated from the normal to the front, a prism sheet forredirecting the light to the front toward the liquid crystal cell isused.

[0038] In this invention, it is important in the principle of thisinvention that the S component is reflected in the interface of thelight guide layers. The number of the interfaces; i.e., the number ofthe light guide layers can be reduced by causing as much S component aspossible to be reflected to reduce the S component transmitting throughthe interface.

[0039] This invention provides a conversion efficiency comparable tolight guide layers using an isotropic material with a lesser number oflight guide layers by using a material of an anisotropic index ofrefraction as the light guide layers to improve the reflectance of the Scomponent in the interface. The axes of two indices of refraction of theanisotropic material coincide with the planes of P and S components,respectively. While the index of refraction in the direction of the axislying in the plane of the P component may be the same as a conventionalone, the index of refraction in the plane of the S component is higherthan the conventional one. The higher, the better. It is seen from theexpression of the reflectance Rs described above that the reflectance ofthe S component becomes larger when the index of refraction in the axisof the plane of the S component in the interface is larger.

[0040] The light guide unit comprising laminated light guide layers ofsuch anisotropic index of refraction receives an incident light from alight source at the end surface thereof which is a cross-section of thelaminated layers to cause a part of the incident light to be reflectedat the opposite end surface which is obliquely cut and the rest of thelight to be transmitted therethrough. A quarter wave length plate isattached to the obliquely cut end surface and a reflecting plate isprovided under the wave length plate.

[0041] The light transmitting through the end surface is reflected bythe reflecting plate after being rotated by the wave length plate and isincident to the end surface after being rotated by the wave length plateagain. The light incident to the end surface is incident to theinterface where it is transmitted and reflected as described herein.However, the majority of the S component is reflected in the interfacewith the rest transmitting through the interface in this invention.Therefore, the light from the light source can be converted to the Pcomponent with a lesser number of layers.

[0042] In this invention, it is preferred that the light incident to thefirst interface of the light guide is incident in the Brewster angle. Itis readily seen by drawing a geometrical drawing that the angle ofincidence of the light to the obliquely cut end surface of the lightguide unit decides the angle of incidence at the interface. In thisinvention, the angle of incidence of the light to the obliquely cut endsurface of the light guide unit is so adjusted that the light incidentto the first interface of the light guide is incident in the Brewsterangle.

[0043] The light guide unit is so inclined with respect to the wavelength plate and the reflecting plate as to provide an incident angledecided in this manner. In order to reduce the inclination, a pluralityof slopes making such incident angle may be formed in the obliquely cutend surface. This allows a necessary incident angle to be providedwithout inclining the entire light guide unit in this angle. This allowsthe thickness of the entire unit to be further reduced.

[0044] In this invention, the light guide unit may be formed into ashape of a triangular wedge consisting of the top layer of the laminatedlayers, the obliquely cut end surface and the surface to which the lightfrom the light source is incident. This allows a wedge-shaped space tobe provided under the unit for receiving various components. This isadvantageous for a portable data processing device in which a thin andlight weight type is especially desired.

[0045] In another aspect of this invention, the prism means fordirecting the polarized light to the front has a plurality of prismsdisposed in a same pitch as columns of the liquid crystal cells. Eachprism has an incident surface and a reflecting surface. Because thelight is emitted from the reflecting surface, a portion corresponding tothe incident surface is dark. In this invention, the dark incidentsurface portion is so disposed as not to contribute illuminating theliquid crystal cell by having the portion corresponding to thereflecting surface align the columns of the liquid crystal cell. All thepolarized light emitted from the light guide is thus directed to theliquid crystal cell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0046]FIG. 1 is a schematic diagram showing a conventional LCD device.

[0047]FIG. 2 is a diagram showing the structure of a conventional LCDpolarizer plate unit.

[0048]FIG. 3 is a diagram showing the structure of a conventional LCDpolarizer plate unit.

[0049]FIG. 4 is a diagram showing refraction of light between differentmaterials.

[0050]FIG. 5 shows a characteristic plot of reflectance when the lightis incident from a material having an index of refraction of 1.0 to amaterial having an index of refraction of 1.49.

[0051]FIG. 6 shows a characteristic plot of reflectance when the lightis incident from a material having an index of refraction of 1.49 to amaterial having an index of refraction of 1.0.

[0052]FIG. 7 is a diagram showing deflection of the light by a prismsheet.

[0053]FIG. 8 is a diagram showing deflection of the light by a prismsheet.

[0054]FIG. 9 is a schematic diagram showing a concept of anotherembodiment of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0055] The basic structure of this invention is shown in FIG. 2. Thelaminated light guide unit is made of thin light guide layers laminatedas shown and a light source is attached to an end surface thereof. Thelight source comprises a fluorescent lamp and a reflecting sheet. Thelamination is cut so as to assume an oblique end surface to which acombination of a quarter wave length plate and a reflecting plate isaffixed.

[0056] The end surface makes an angle φ with respect to the quarter wavelength plate and the reflecting plate. Therefore, the light is incidentto the end surface at an angle of π/2−2φ. It is readily found from ageometrical drawing that an incident angle to the interface betweenlayers is π/2−2φ.

[0057] It is preferable that this incident angle is the Brewster angle.When the light is incident to the interface at the Brewster angle, allpolarization component lying in the incident plane (P) transmits theinterface while all polarization component lying in a plane orthogonalto the incident plane (S) is reflected. Any S component which maytransmit through the interface will be reflected by the next interface.The S component reflected from the interface is returned back to thelight guide layers by the wave length plate and the reflecting plate asa P component and incident again to the interface. The S component isreflected each time the above process is repeated at a plurality ofinterfaces transmits through the interface as a P component so that alarge portion of the light from the light source is emitted from thelight guide unit as a P component.

[0058] A plurality of thin light guide films may be further laminated onthe top light guide layer of the light guide unit as shown in FIG. 3.This further adds interfaces of transmission and reflection.

[0059] The light guide member and the plurality of light guide layersare preferably of a material which assumes a low internal absorption ofthe light, such as an acrylic sheet and preferably transparent materialsincluding acrylic resin, PMMA (polymethylmethacrylate), polycarbonate,polyethylene, Se, and AgCl. The shape of the light guide member may bein a shape suitable for use such as a bar and a curved surface withoutbeing limited to a plate and a sheet.

[0060] The light guide member may be of a single piece or a laminationof a plurality of sheets. These light guides are not limited to a samesize or a same material and a member requiring stiffness may be designedthick while a member which does not require stiffness may be designedthin. Also, materials of differing indices of refraction may bedeposited in multiple layers on a stiff light guide to increase thenumber of laminated layers while maintaining a stiffness.

[0061] In using an acrylic sheet in the light guide member, thethickness of the sheet is preferably 0.1 to 4.0 mm from theconsideration of the stiffness and the efficiency of light utilization.The lamination as used in this invention is not limited to insertion ofair between the light guides and water vapor may be introduced betweenthe guides for preventing degradation of the light guide unit, water oran adhesive may be inserted between the guides for preventing the guidesfrom being peeled off, or a material having an index of refractiondiffering from the light guide may be inserted. Higher reflectance ofthe reflecting plate is preferable in this invention and the reflectingplate may be made of an aluminum deposited sheet, a silver depositedsheet and a metal foil, etc.

[0062] In this invention, the light guide layer is made of a materialhaving a high index of refraction in the axis lying in the plane of theS component. For example, while the isotropic index of refraction of anacrylic material is normally 1.49, the index can be increased up toabout 1.69 in the direction of the axis lying in the plane of the Scomponent. By doing so, an increased portion of the S component isreflected in the interface (lesser amount of the S component transmitthe interface) so that an unchanged effect can be resulted with a lessernumber of layers than those required for an isotropic material.

[0063] For example, when an acrylic material having an index ofrefraction 1.49 is used as an isotropic material, the reflectance of theS component is 28% while the transmittance is 72% per layer. With tenlayers laminated, the overall transmittance will be 0.72¹⁰=0.04.

[0064] On the other hand, if the index of refraction in the direction ofthe S component is 1.66, the reflectance is 40% while the transmittanceis 60% and the same effect is obtained with 6 layers (0.6⁶=0.04). Asheet having such anisotropic index of refraction is easily available inthe market.

[0065] While the thickness of the light guide film is not important andit is preferable that the number of the interfaces is as large aspossible, the light guiding layer is preferably as thin as possible fromthe view point of reducing the weight of the light guide unit. An extraspace is created by making the thickness of the light guiding layer inthis portion extremely thin and the layers of substantially same sizemay be used in lamination without requiring the layers to beprogressively in different sizes resulting in a stepped structure asshown in FIG. 2.

[0066] As such, the light is not lost by re-entering from the edge ofthe layers and dark and bright stripes are eliminated. Even if the stepsremain in the layers as shown in FIG. 2, there is little possibility ofthe light re-entering and recognizable stripes are not generated becausethe layer is thin and the size of the edge is very small.

[0067] By employing the above structure, this invention allows thecross-sectional shape of the light guide unit to be of a triangularshape as shown in FIG. 3, in contrast to the conventional unit which hada rectangular cross-section as shown in FIG. 2. By this structure, theweight and the volume of the unit can be about half the conventionalunit. Also, this invention can implement a mode which is similar to themode in which a conventional back light (not generating a polarizedlight) uses a light guide of a wedge shaped cross section to provide aneffective use of a space and allows a conventional back light to bereplaced with the present polarized back light in a form compatible withthe conventional type.

[0068] While the light guide layer is acrylic material and thesurrounding material is air in the example so far described, anymaterial of the layer and any surrounding material may be used so longas the indices of refraction of the materials allow the incident lightto satisfy the Brewster angle or an angle which is near the Brewsterangle.

[0069] The following condition is required for the incident angle π/2−2φto be the Brewster angle θ_(B). In the expression, n₁ is an index ofrefraction of the light guide, n₂ is an index of refraction of amaterial other than the light guide (air in FIG. 5), and φ is the angleof the groove (the slope of the larger angle of inclination). Therelationship between Brewster angle θ_(B) and n₁, n₂ is given by;

θ_(B)=sin⁻¹ [n ₁ ²/(n ₁ ² +n ₂ ²)]^(½) (rad)

[0070] The angle of incidence to the upper surface of the light guide isgiven by a geometric analysis using φ;

π/2−2φ (rad)

[0071] Snell's law is expressed on the upper surface of the light guideas;

sin θ_(B)/sin (π/2−2φ)=n ₁ /n ₂

[0072] solving this expression for φ gives the following generalsolution;

φ=cos⁻¹ {(n ₂ /n ₁) [n ₁ ²/(n ₁ ² +n ₂ ²)]^(½)}/2 (rad)

[0073] Any medium satisfies the condition of this invention so long asit satisfies the above general expression.

[0074] While the entire light guide unit is inclined with respect to thewave length plate and the reflecting plate so as to provide an incidentangle which is equal to the Brewster angle, many sloped surfaces whichprovide such incident angle can be formed in the obliquely cut endsurface. As shown in the enlarged view in FIG. 3, many sloped surfacesrunning perpendicularly to the face of the drawing are formed in theobliquely cut end surface and are so disposed as to provide a desiredangle to the incident light in the light guide. An incident anglesatisfying the Brewster angle is thus provided though the entire lightguide is not inclined in this angle. A necessary incident angle can bethus provided while the light guide unit is not entirely inclined inthis angle thereby reducing the thickness of the entire unit.

[0075] This invention is contemplated for use as a back light of aliquid crystal display device. The liquid crystal display devicecomprises a light source and glass substrates sandwiching a liquidcrystal to which a polarized light emitted from the light guide unit ofthis invention is incident.

[0076] The light emitted from the light guide is largely inclined in70-degrees from the front thereof in this invention. Two methods areavailable for deflecting the light to the right angle to the frontsurface. The first method is to have the light refract twice to deflectit to the front, in which a prism sheet is used with the apex thereoforiented upward as shown in FIG. 8. When the index of refraction n ofthe material of the prism is 1.58, a prism sheet having an angle of apexof 32-degrees is required to deflect the light to the front.

[0077] A second method is to have the light refract once and totallyreflect once to deflect to the front, in which the prism sheet is usedwith the apex thereof oriented downward as shown in FIG. 9. In thiscase, a prism having an angle of apex of 65.4-degrees is required. Asseen in the above, a same effect is resulted whether the prism isoriented upward or downward. From the view point of fabrication, it ismore advantageous in the view point of yield and cost to use the prismwith the apex oriented downward because a smaller apex angle of a prismis more difficult to fabricate (a larger apex angle can be used when theapex is oriented downward). The prism sheet is made of a glass orplastic material.

[0078] In FIGS. 7 and 8, it is seen that the sloped surface of eachprism which is not the light reflecting surface does not emit the lightto the front. In other words, the light emitted from the prism sheet isin a stripe pattern. This may possibly induce an interference patternwith a gate line or a data line of the liquid crystal cell. In order toprevent the stripe pattern from being generated, the pitch of the prismsof the prism sheet (50 microns, for example) can be made smaller thanthe pitch of the liquid crystal cell (200 microns, for example) tomismatch the pitches. By doing so, the prism sheet is observed as if itemits the light uniformly from the front and the interference patterncan be prevented from being generated because the pitch of the prismsheet is very small.

[0079] However, the light incident to a portion of the liquid crystalcell array which has no opening is absorbed there and wasted in thiscase. Another aspect of this invention provides a structure in whichsuch waste is avoided.

[0080] According to this structure of this invention, the pitch of theprisms of the prism sheet is made the same as the pitch of the liquidcrystal cell array so that the opening part of the liquid crystal cellcoincides with a portion of the prism corresponding to the reflectingsurface which emits the light. The portion corresponding to the slope ofthe prism which is not the reflecting surface coincides with the parthaving no opening.

[0081]FIG. 9 is a schematic diagram showing a concept of the inventivestructure. As shown in FIG. 9, the pitch of the prisms of the prismsheet is made the same as the pitch of the liquid crystal cell array.The light reflected by the reflecting surface of the prism is directedto the opening part of the liquid crystal cell. The part having noopening does not receive the light because it faces to the surface whichis not a reflecting surface. All the light emitted from the light guideunit is thus directed to the opening part, and there is no light whichis absorbed without being utilized. It is easy to manufacture the prismbecause the prism has a larger pitch than those shown in FIGS. 7 and 8.The apex angle and the ratio of reflecting/transmitting surfaces may besuitably decided in a specific design work.

[0082] As shown in FIG. 9, the liquid crystal cell array may be formeddirectly on the prism sheet. In this case, the prism sheet also plays arole of a glass substrate of the liquid crystal cell. The number ofinterfaces between media is decreased by 2 when compared to a case wherean independent prism sheet is disposed between the liquid crystal celland the light guide film, resulting in a corresponding improvement ofefficiency.

[0083] The thickness and the weight of the light guide are reducedbecause the light is converted to the P polarized component with thenumber of layers less than those of a conventional light guide accordingto this invention. There is no light which is absorbed without beingutilized in another aspect of this invention because all the light fromthe light guide is directed to the opening part of the liquid crystalcell.

[0084] The following is a brief description of the reference numbers asused in the drawings:

[0085]100: Conventional LCD device

[0086]101: Light source

[0087]102: Light guide plate

[0088]103: Diffusion sheet

[0089]104: Lower polarizer plate

[0090]105: Glass substrate

[0091]106: Color filter

[0092]107: Upper polarizer plate

[0093]108: Back light

[0094]201: Material 1

[0095]202: Material 2

[0096]203: Interface between the materials

[0097]204: Incident light

[0098]205: Reflected light

[0099]206: Transmitted light

[0100] While the exemplary preferred embodiments of the presentinvention are described herein with particularity, those having normalskill in the art will recognise various changes, modifications,additions and applications other than those specifically mentionedherein without departing from the spirit of this invention.

What is claimed is:
 1. A light guide device comprising: a light guide unit consisting of a lamination of a plurality of light guide layers in which one end thereof is a light incidence surface and the other end is cut obliquely with respect to the direction of the lamination; a reflecting plate disposed adjacent to said other end surface; and means disposed between said other end surface and said reflecting plate for changing the polarization direction of light, said light guide layers having an anisotropic index of refraction in which the index of refraction in the axis of reflecting one of the polarized components of the light from said incident surface is larger than the index of refraction in the axis of transmitting the other one of said polarized components.
 2. A light guide device of claim 1 in which said other end surface is cut obliquely in an angle such that the light reflected by said other end surface and incident to an interface between said light guide layers is incident in the Brewster angle.
 3. A light guide device of claim 1 in which said other end surface has a plurality of sloped surfaces in the direction perpendicular to the direction of the lamination of the light guide layers and the direction of incidence of the light.
 4. A light guide device of claim 3 in which said sloped surface comprises a combination of a surface having an angle of inclination smaller than the angle of inclination of said other end surface and a surface having a larger angle of inclination which is an angle causing the light reflected from the latter surface and incident to the interface between the light guide layers to be incident in the Brewster angle.
 5. A light guide device of claim 4 in which the angle φ of the surface having a larger angle is given by the following expression: φ=cos⁻¹ {(n ₂ /n ₁) [n ₁ ²/(n ₁ ² +n ₂ ²)]^(½)}/2 (rad) where n₁ is an index of refraction of the light guide and n₂ is an index of refraction of a material other than the light guide.
 6. A light guide device of claim 5 in which a plurality of light guide films are further laminated on the top layer of said plurality of light guide layers of said light guide unit.
 7. A light guide device of claim 26 in which said light guide film is thinner than said light guide layer.
 8. A light guide device of claim 6 in which a phase film for changing the direction of polarization is further disposed on said plurality of light guide films.
 9. A liquid crystal display device comprising a liquid crystal cell and a light guide device disposed in the back of said liquid crystal cell, said light guide device comprising: a light guide unit consisting of a lamination of a plurality of light guide layers in which one end thereof is a light incidence surface and the other end is cut obliquely with respect to the direction of the lamination; a reflecting plate disposed adjacent to said other end surface; means disposed between said other end surface and said reflecting plate for changing the polarization direction of a light; and a prism sheet disposed on the top light guide layer of said light guide unit and having an apex part only on one side thereof, the side where said apex is formed facing said light guide unit, said light guide layers having an anisotropic index of refraction in which the index of refraction in the axis of reflecting one of the polarized components of the light from said incident surface is larger than the index of refraction in the axis of transmitting the other one of said polarized components.
 10. A liquid crystal display device comprising a liquid crystal cell and a light guide device disposed in the back of said liquid crystal cell, said light guide device comprising: a light guide unit consisting of a lamination of a plurality of light guide layers in which one end thereof is a light incidence surface and the other end is cut obliquely with respect to the direction of the lamination; a reflecting plate disposed adjacent to said other end surface; means disposed between said other end surface and said reflecting plate for changing the polarization direction of a light, and a prism sheet disposed on the top light guide layer of said light guide unit and having an apex part only on one side thereof, the side where said apex is formed facing said light guide unit; the pitch of a plurality of prisms of said prism sheet is the same as the pitch of the array of the liquid crystal cells.
 11. A liquid crystal display device of claim 10 in which a reflecting surface of each prism of said prism sheet is aligned with an opening part of the liquid crystal cell.
 12. A liquid crystal display device of claim 11 in which said liquid crystal cell is adhered to said prism sheet.
 13. A liquid crystal display device of claims 10 in which said prism sheet is integrally formed with a substrate of said liquid crystal cell.
 14. A liquid crystal display device comprising: a matrix array of liquid crystal cells arranged on a transparent substrate; and a column of a plurality of prisms formed on the back of said substrate with respect to said liquid crystal cells with a reflecting surface of each prism being aligned with an opening part of said liquid crystal cell.
 15. A liquid crystal display device of claim 14 in which said substrate is formed of a glass.
 16. A liquid crystal display device of claim 14 in which said substrate is formed of a plastic material.
 17. A liquid crystal display device of claim 11 in which said array of liquid crystal cells is formed directly on said prism sheet.
 18. A liquid crystal display device of claim 10 in which a polarizer plate is further disposed on said prism sheet.
 19. A liquid crystal display device of claim 10 in which said prism sheet is formed of a polarizing material.
 20. A liquid crystal display device of claim 10 in which said light guide layers has an anisotropic index of refraction in which the index of refraction in the axis of reflecting one of polarized components of the light from said incident surface is larger than the index of refraction in the axis of transmitting the other one of said polarized components.
 21. A liquid crystal display device of claim 17 in which a polarizer plate is further disposed on said prism sheet.
 22. A liquid crystal display device of claims 17 in which said prism sheet is formed of a polarizing material.
 23. A liquid crystal display device of claim 19 in which said light guide layers has an anisotropic index of refraction in which the index of refraction in the axis of reflecting one of the polarized components of the light from said incident surface is larger than the index of refraction in the axis of transmitting the other one of said polarized components. 